In case of the PDC system, as for a conventional antenna for a radio terminal device, a diversity antenna in combination of a whip antenna 102 and a planar inverted F antenna 103, as shown in FIG. 20, has been used well for dealing with a fading problem. However, all of these antennas have the polarization in which the z-axis directional component is big. Therefore, in a radio communications system transmission/reception in the vertical polarization at the base station, when a portable phone is used for telephone call and data communications as shown in FIG. 21, the polarization loss becomes small at the time of data communications as the polarization is the same vertical polarization, while the polarization loss becomes great depending on the slope of a plane of polarization at the time of telephone conversation.
Accordingly, in any states such as telephone call or data communications, there was described an antenna from which good polarization characteristics can be obtained, for example, in JP-A-2001-326514. FIG. 22 illustrates a conventional radio terminal device as described in the aforementioned patent publication.
In FIG. 22, a radio terminal device has one or a plurality of conversion switches 2202 for switching electrical connection state on an antenna element 2201 in which the electrical length is approximately 1 wavelength for a frequency in use. By switching the conversion switches 2202, the main polarization characteristic of an antenna can be switched to the horizontal polarization and vertical polarization. Meanwhile, a conversion control circuit section 2203 is provided with a discriminating means of a communication media such as voice communications/data communications or a discriminating means to decide whether any external device is connected to the radio terminal device. By switching the conversion switch 2202 based on the discrimination results from such discriminating means, the user is able to have a good plane of polarization automatically without awareness.
Meanwhile, there was mentioned a technique to switch the polarization and the directivity of an antenna by switching an antenna to the balanced characteristic and unbalanced characteristic in JP-A-2002-43826. FIG. 23 illustrates a conventional radio terminal device as described in the aforementioned patent publication.
In FIG. 23, the radio terminal device is provided with a rectangular loop antenna element 2303 of 1 wavelength arranged in the proximity of a radio substrate 2301 and both ends of the loop antenna element 2303 are bent so as to face toward a feeding section, thus configuring the current distribution that a current in the most front end portion becomes zero. Also, by concentrating the current on the loop antenna element 2303, the current component flowing on the radio substrate 2301 is reduced and the influence when a human being holds the radio terminal device in the hand is suppressed, and the directive characteristic is formed according to the arrival wave at the same time. Furthermore, by adjusting a phase circuit 2304, it is possible to switch the balanced characteristic and the unbalanced characteristic or to have a state therebetween and to form a plurality of radiation directional patterns in one antenna system depending on the environment of use or an arrival radio wave.
Meanwhile, other than the above JP-A-2002-43826, there was mentioned about controlling the directivity of an antenna by changing the current distribution in JP-A-2001-223514. FIG. 24A and FIG. 24B show a configuration of a case current distribution control section of the conventional radio terminal device as described in this patent publication.
In FIG. 24A, in the case current distribution control section, metal plates 2401 such as an case in a plurality of shapes of strips having a predetermined width are arranged at predetermined intervals, and connected by means of devices such as a plurality of diodes 2402 or the like therebetween. Then, by turning ON/OFF the switch 2404 of a bias control circuit 2403, voltages applied to diodes 2402 are controlled and the current distribution is switched.
However, in the conventional method as described in JP-A-2001-326514, there were problems in that an antenna element of approximately 1 wavelength was needed, which could not be installed at the case in view of a configuration of an antenna so that it was difficult to produce a small-sized terminal. Meanwhile, the conversion switch 2202 served as a means to switch an impedance matching circuit and switching at the same frequency band was not considered.
Meanwhile, according to the conventional method as described in JP-A-2002-43826, a small-sized antenna could be produced by using a folded loop antenna, but there was a problem in that the balanced characteristic and unbalanced characteristic could not be switched at the same frequency band as they were switched at the phase circuit.
Also, as for the conventional method as described in JP-A-2001-223514, there was a problem in that many devices such as diodes and the like were needed for the control of the current distribution.